The present invention relates to data transmission circuits, and more particularly to circuits for passing signals across voltage differences. For example, to monitor the state of circuits operating at respective voltages, data exchanges with the circuits must account for the different operating voltages of the respective circuits. One example of an area that this issue presents itself in is monitoring the state of cells in a set of batteries. For example, the net terminal voltage of a set of batteries may be in the range of 15 v to 100 v. In such a case, there may be, for example, twenty-five batteries in the set. Obviously, the voltages at the individual cells increase toward the terminal voltage of the set. Accordingly, assessing the voltage of an individual cell must account for the voltage of that cell with in the set.
There are several ways to address the voltage differential issue. A first approach is to use a common mode rejection resistor network. The voltage of a cell can be monitored using a buffer amplifier and analog multiplexer together with an analog-to-digital converter. This approach, however, typically requires the use of a laser trimmed resistor network. This adds to the expense of the circuit, and requires additional manufacturing time and equipment. Also, for more than four or five cells, it is difficult to achieve the desired resistor network accuracy, even with laser trimming. Even if the desired accuracy could be achieved, the resistor network draws power from the cells. This drains the cells.
Another approach to monitoring the voltage of individual cells in the battery set is to use common mode rejection signal sampling transformers. These transformers can present selected measurements to a single analog-to-digital converter. The use of transformers increases the size, cost, and weight of the circuit.
A third approach to monitoring the voltage of individual cells in the battery set is to use individual analog-to-digital converters. The outputs of the analog-to-digital converters are multiplexed to provide the voltage of the desired cell. This approach has distinct advantages, one of which is compact analog-to-digital converters. The difficulty is, however, in implementing an acceptable multiplexing scheme to output the multiplexed signal.
There are variations on the third approach. One is to have the operational control of the analog-to-digital converters and the output multiplexing accomplished with a MIL-STD-1553 data bus. This data bus crosses voltage differentials through the use of transformers, which have the previously mentioned size, cost, and weight impacts. Another variation on the third approach is to have the operational control and the multiplexing implemented using opto-isolators. However, opto-isolators draw too much current and are not compatible with integration of a suitable circuit on an integrated circuit.
Another variation on the third approach is to use frequency signals applied through DC isolating capacitors for the purpose of operational control and multiplexing control. It is difficult, however, to make capacitors of substantial size internal to an integrated circuit. So, using capacitive isolation raises manufacturing costs in a manner similar to that encountered when using transformers. This approach adds complexity to the interface circuitry, and tends to cause electromagnetic interference. Avoiding such interference commonly requires bulky shielding, which complicates and increases the cost of packaging.
It is an object of the present invention to provide an inexpensive structure for passing signals across voltage differences.
It is another object of the present invention to provide a simple structure for passing signals across voltage differences.
It is a further object of the present invention to provide a structure for passing signals across voltage differences that can be fabricated as an integrated circuit.
It is still another object of the present invention to provide a structure for passing signals across voltage differences that does not rely on signal transformers.